Sensor dust shows as dark specks in exactly the same place in every photograph. That repetition is the diagnosis: a mark that moves is on your screen, and a mark that stays is on the sensor.

Find out whether you have any

Set the smallest aperture your lens offers — f/16 or f/22 — and photograph something plain and bright. A white wall, an overcast sky, a sheet of paper.

Look at the result at full size. Dust shows at small apertures and disappears at wide ones, which is why it is invisible in most photographs and obvious in a landscape at f/16.

Try the camera’s own cleaning first

Almost every camera has a sensor-cleaning routine that vibrates the filter in front of the sensor. It is free, carries no risk, and shifts a useful proportion of what lands there. Run it a few times before doing anything else.

Then a blower, and nothing else

Lock the mirror up or enter sensor-cleaning mode, hold the camera face down, and use a hand blower without letting the tip enter the chamber.

Do not use canned air. The propellant can reach the sensor and it is very difficult to remove.

Wet cleaning is a last resort

Swabs and fluid work, and they also carry real risk of scratching the filter. If the marks survive the blower and they genuinely bother you, that is the point to decide between doing it carefully yourself and paying someone.

Preventing most of it

Change lenses with the camera pointing down, so gravity is working for you.
Turn the camera off first; a powered sensor holds a static charge that attracts dust.
Have the replacement lens ready before the first one comes off.
Avoid changing lenses in wind.